In depositing various combinations of strike and final deposit, it has often been proposed that different compositions for the strike bath and the final bath be used, e. g., a low current density, i. e., low speed bath or a high speed bath. As a result, the components from the strike bath are either dragged into the plating bath or precautions must be taken to insure that adequate rinsing or treatment has taken place. Consequently, additional process steps and more elaborate methods for assuring purity and non-staining of the final product are required and attendant increase in cost has often been sizable.
As most strike and silver plating baths are based on high cyanide content, for this reason, in general, a strike bath employed for silver electroplating in the prior art has been run at an alkaline pH. These baths contain silver on elemental basis at high concentrations.
Other baths are such as disclosed in U.S. Pat. No. 2,504,272 for deposition of silver from a pyrophosphate solution using silver pyrophosphate in combination with ammonium ions to form a soluble complex source of silver. Although a number of silver salts as a source of silver ions have been disclosed, the above-mentioned patent does not disclose the presently claimed combination for either strike, low speed or high speed. While a prestrike or pretreatment plating has been disclosed, the silver deposit is from a bath of relatively low silver concentration and using ammonium ions to complex silver as a source of silver ions.
Further, in U.S. Pat. No. 3,427,232 a process is described for electroplating silver, but in this prior art the use of silver cyanide bath is deprecated as not being useful for plating, especially on the particular magnesium substrate. Inasmuch as the disclosed combination is different and suggests not to combine a silver cyanide bath with the disclosed bath, the method disclosed in this patent is inapplicable with respect to the prestrike, or final deposit combination disclosed herein.
In U.S. Pat. No. 2,435,525, a disclosure of a prestrike has been found which shows a copper cyanide bath used for depositing copper and thereafter the substrate is rinsed and dipped in a cyanide solution free of copper and silver ions. A silver strike is then applied on the copper strike, employing a bath of low current density after which the work piece to be plated is transferred to a silver plating bath operated at a higher current density. The strike bath as well as the plating bath, however, have a different combination of elements and these provide for a very high pH such as between 12.6 and 13.4.
Other strike and final deposit combinations have been shown in U.S. Pat. Nos. 2,493,092 and 3,329,522. However, the deposits which have been obtained are based on different metals, and these baths are not applicable with respect to silver plating.
Another cyanide free succinic acid (hence an organic) bath has been disclosed in vague terms in Chemical Engineering, Sept. 15, 1975, on page 68.
In view of the above, it is readily evident that a bath which has been used in the prior art for final plating seldom, if ever, corresponds to a strike bath or vice-versa; the combination of the elements in each bath is different as well as the components used therewith. Moreover, a bath having as low a silver concentration as for the present baths which is essentially at neutral pH and operates substantially without free cyanides and consists essentially of only inorganic constituents has not been described in the prior art according to my knowledge.